In recent years, thermal transfer systems have been developed to obtain prints from pictures which have been generated electronically from a color video camera. According to one way of obtaining such prints, an electronic picture is first subjected to color separation by color filters. The respective color-separated images are then converted into electrical signals. These signals are then operated on to produce cyan, magenta and yellow electrical signals. These signals are then transmitted to a thermal printer. To obtain the print, a cyan, magenta or yellow dye-donor element is placed face-to-face with a dye receiving element. The two are then inserted between a thermal printing head and a platen roller. A line-type thermal printing head is used to apply heat from the back of the dye-donor sheet. The thermal printing head has many heating elements and is heated up sequentially in response to one of the cyan, magenta or yellow signals, and the process is then repeated for the other two colors. A color hard copy is thus obtained which corresponds to the original picture viewed on a screen. Further details of this process and an apparatus for carrying it out are contained in U.S. Pat. No. 4,621,271, the disclosure of which is hereby incorporated by reference.
Dye donor elements used in thermal dye transfer generally include a support bearing a dye layer comprising heat transferable dye and a polymeric binder. Dye receiving elements generally include a support bearing on one side thereof a dye image-receiving layer. The dye image-receiving layer conventionally comprises a polymeric material chosen from a wide assortment of compositions for its compatibility and receptivity for the dyes to be transferred from the dye donor element. The polymeric material must also provide adequate light stability for the transferred dye images. Many of the polymers which provide these desired properties, however, often lack the desired strength and integrity to stand up to the rigors of thermal printing. For example, a significant problem which can be encountered during thermal printing is sticking of the dye donor to the receiver. Gloss and abrasion resistance may also be marginal with many receiving layer polymers.
Increasing the hardness of the receiver layer with polymers having higher glass transition temperatures (Tg) can improve physical properties, but penetration of the dye into such layers may be impaired.
An alternate approach to achieve improved film properties is to crosslink the polymer. Crosslinking may be achieved in a variety of different ways, including reaction curing, catalyst curing, heat curing, and radiation curing. In general, a crosslinked polymer receiver layer may be obtained by crosslinking and curing a polymer having a crosslinkable reaction group with an additive having a crosslinkable reaction group, as is discussed in EPO 394 460, the disclosure of which is incorporated by reference. This reference, e.g., discloses receiving layers comprising polyester polyols crosslinked with multifunctional isocyanates. While such crosslinked polyester receiving layers are generally superior in resistance to sticking compared to non-crosslinked polyesters, light stability for transferred image dyes may still be a problem.
U.S. Pat. No. 5,266,551 relates to a dye-image receiving layer for thermal dye transfer wherein the receiving layer comprises a crosslinked polymer network formed by the reaction of multifunctional isocyanates with polycarbonate polyols having two terminal hydroxy groups. However, there is a problem with this dye image-receiving layer in that images transferred to it have low density.
U.S. Pat. No. 4,871,715 relates to dye image-receiving elements containing phthalate esters in the receiving layer. However, there is a problem with these plasticizers in that the light fade of the transferred image is not as good as one would like.
It is an object of this invention to provide a dye image-receiving element for thermal dye transfer processes having improved dye uptake and image stability. It is a further object of the invention to be able to coat such a receiving layer with a minimum amount of chlorinated solvent.